Climategate E-mails show that climate scientists wanted to get rid of the 1940 temperature spike. Part of that strategy was to invert it in the Southern Hemisphere, which they did.
From: Tom Wigley <firstname.lastname@example.org>
To: Phil Jones <email@example.com>
Date: Sun, 27 Sep 2009 23:25:38 -0600
Cc: Ben Santer <firstname.lastname@example.org>
It would be good to remove at least part of the 1940s blip, but we are still left with “why the blip”.
The 1990 IPCC Report showed that the Southern Hemisphere cooled from 1948 to 1966, but GISS has reversed that into a strong warming trend.